You must have python 3 installed in order for this plugin to work. You must also enable python plugin support in Tiled.
Refer to the Tiled official documentation for more info on installing pyhton scripts as plugins.

On windows the final result should look like C:\Users\{Your Username}\.tiled\metatiles_nes.py

On Linux the final result should look like ~/.tiled/metatiles_nes.py